1. Field of the Invention
The present invention relates to an image forming apparatus, such as a laser printer, for forming images by a laser beam responsive to image information applied from the host unit.
2. Description of the Related Art
In the case of the conventional laser provided in a laser printer, a laser beam detecting signal applied from a laser beam detecting sensor every time a horizontal scanning operation is carried out is used as a horizontal synchronizing signal, and a timer is made operative for a predetermined time period and synchronizes with the front edge of the laser beam detecting signal, while the output laser beam is turned off. When the operation of the timer is finished, the output laser beam is again turned on and a next laser beam detecting signal is again created.
Conventionally, the laser beam detecting signal is used directly as a trigger signal for starting the timer. In the case where the laser beam detecting sensor had stains and flaws on its light-receiving face, therefore, the waveform of the laser beam detecting signal is split to thereby generate two horizontal synchronizing signals, for example, at the time one scanning operation is finished.
Further, the output laser beam is turned off, synchronizing with the front edge of the laser beam detecting signal. Therefore, the width of the horizontal synchronizing signal became extremely narrow, like whiskers. When a signal transmitting line is long, therefore, the horizontal synchronizing signal is lost, because of electrostatic stray capacitance on the signal transmitting line.